Collinear termination for high energy particle linear accelerators



Aug. 2, 1966 J. HAsMsoN 3,264,515

COLLINEAR TERMINATION FOR HIGH ENERGY PARTICLE LINEAR ACGELERATORS FiledJune 29, 1961 :o a II l2 l3 I4 l5 2 r L. 4. ,7 ,7, i l6 kW 5 X! 2 Y-1IZ-| sum INPUT BUNCHING UNlFORM ACCELERATING TERMINATING COUPLER SECTIONSECTIO 3SECTION 15 FE. f

INVENTOR. JACOB HAIMSON ATTORNEY United States Patent 3,264,515COLLINEAR TERMINATION FOR HEGH ENERGY PARTICLE LINEAR ACCELERATORS JacobHaimson, Palo Alto, Calif., assignor to Varian Associates, Palo Alto,Calif., a corporation of California Filed June 29, 1961, Ser. No.120,597 12 Claims. (Cl. 3155.41)

The present invention relates in general to particle accelerating meansand more specifically to a method and apparatus for attenuating radiofrequency waves at the end of the particle accelerating structures suchas linear accelerators.

In linear accelerators a source of particles such as an electron beam isdirected down a slow wave structure which is adapted to propagate aradio frequency electromagnetic wave. The dimensions of the waveguidestructure can be arranged such that there is an interaction between thetraveling radio frequency wave and the electron beam whereby energy istransferred from the wave to the beam to accelerate electrons in thebeam. At the end of the accelerating guide the accelerated electron beamcan be directed onto an X-ray target for producing X- rays or can bepassed out of the vacuum envelope and directed onto the object beingirradiated. In optimizing the design of one type of acceleratingstructure which is to be utilized for X-ray production it has been founddesirable to arrange for approximately ten percent of the radiofrequency power to be remanent at the end of the accelerating waveguideafter allowance for dissipation losses and beam loading. In otherapplications greater or less residual radio frequency power may resultdue to a variety of designs and operational requirements. For example, agreater amount of residual radio frequency power could be due to reducedbeam loading caused by a reduction in beam peak current or the dephasingof the radio frequency wave from a synchronous condition in order toreduce the terminal energy of the electrons. Less residual radiofrequency power could result from in creased beam loading. In particleaccelerators for X-ray generation it is sometimes desirable to have evengreater percentages of residual power at the end of the acceleratingwaveguide in the interest of output stability. This residual power isextracted from the accelerating waveguide and is either applied to anexternal load or fed back to the input of the accelerator. Suchstructures required a radio frequency transition at the end of theaccelerating structure and may also require either an external load fordissipating the power or means for directing the radio frequency powerback to the input. Also a radio frequency wave permeable window may benecessary.

As well as involving a great deal of expense, the additional structurenecessary to dispose of the radio frequency power requires aconsiderable amount of space. In many applications it is necessary tohave the particle accelerator fit in as small a space as possible. As atypical example, it is desirable to have particle accelerators which areused for therapy fit within as small a room as possible so that theaccelerator can be placed in an existing room in a hospital. Since it isdesirable to have the particle accelerator mounted on a gantry forirradiating a patient from many angles, the necessity for space aroundthe end of the accelerating structure for terminating the radiofrequency wave prohibits the use of a minimal size gantry.

Also when the residual radio frequency power is extracted at the end ofthe particle accelerator, the particle beam focusing structure iscaptured between the radio frequency wave coupling structures which arepositioned at the ends of the accelerating structures. Since it isdesirable to provide a vacuum envelope around the accelerating structureand inside the focusing structure, an assembly and maintenance problemexists. It is usually not possible to complete the vacuum envelope untilthe focusing structure is positioned around the accelerating structure.This vacuum envelope may have to be opened in order to remove thefocusing structure for repairs.

According to the present invention, a termination is provided around thebeam axis at the output end of a particle accelerator guide capable ofdissipating the residual radio frequency power while permitting thetransmission of the particle beam therethrough either without extractingenergy therefrom, without altering the beam energy or by deliberatelyextracting energy therefrom. Such a structure is extremely useful inenvironments where cost and space are at a premium. Such a terminationcan be referred to as a collinear termination since it lies in astraight line with the accelerating section.

Therefore, the principal object of the present invention is to provide atermination around the beam axis at the output end of a waveguidestructure for propagating a radio frequency wave and capable ofdissipating radio frequency power propagating therealong whilepermitting transmission of the electron beam therethrough.

One feature of the present invention is the provision of a resonantsection of loaded Waveguide at the end of a radio frequency, phasevelocity of light structure wherein the residual radio frequency powerat the end of the slow wave structure can be attenuated in accordancewith design requirements.

' Another feature of the present invention is the provision of a radiofrequency wave propagating structure including a waveguide structurecomprising a plurality of coupled cavities formed by a series of discsinterleaved between a series of hollow cylindrical spacers, at leastcertain of the spacers and/or discs including a material having a highelectrical resistivity.

Another feature of the present invention is the provision of a novelstructure of the last aforementioned feature wherein the last cavity ofsaid structure is resonant and adapted for reflecting any radiofrequency wave power at the end of said structure, whereby the radiofrequency power is attenuated in the forward and backward directions.

Another feature of the present invention is the provision of a radiofrequency wave propagating structure comprising a loaded Waveguide atleast a portion of which is provided with an uneven surface, forexample, rough surface finish, for attenuating a wave propagatingtherein.

Another feature of the present invention is the provision of a particleaccelerator structure including an accelerating section and aterminating section, both the accelerating and terminating section beingcomprised of loaded waveguide defining a plurality of cavity resonators,the cavity resonators of said terminating section having a much lower Qthan the cavity resonators of said accelerating section.

Another feature of the present invention is the provision of a particleaccelerator structure including an accelerating section and aterminating section, 'both the accelerating and terminating sectionsbeing comprised of loaded waveguide defining a plurality of cavityresonators,

prised of disc loaded waveguide, the apertures in the discs of theterminating section being smaller than the apertures in the discs of theaccelerating section.

The. accelerating section;14 is typically a length of disc-loadedwaveguide-forming a p'luralityof' coupled cavities wherein the :discshave a constant size aperture therein and are uniformly spaced along thelengthof the Waveguide. In the buncher section:13 the electrons in thebeam are bunched, andthese bunches ride up to the Still another featureof the present invention is "the provision of a particle acceleratingstructure including an accelerating section and a terminating section,both the accelerating and terminating sections comprised of disc loadedwaveguide, the thickness of the discs of the terminating section beinggreater than the thickness of the discs of the accelerating section.

Other features and advantages of-the present invention Will become moreapparent upon a perusual of the following specification'taken inconnection with the accompanying drawing wherein:

FIG. 1 isa schematic view of a particle accelerator utilizing thefeatures of the present invention,

FIG. .2 is an enlarged side view, partially broken away, of a portion ofthe structure shown in FIG. 1,

FIG. 3 is a cross sectional view of a portion of the structure shown inFIG. 2 taken along the line 33 in the direction of the arrows,

FIG. 4, is a side view, similar to a portion of FIG. 2, of analternative embodiment of the present invention,

FIG. 5 is a side view, similar to FIG. 4, of another alternativeembodiment of the present invention, and

FIG. 6 is a side view, similar to FIG. 4, of still another alternativeembodiment of the present invention.

Referring now to FIG. 1 of .the drawing, there is shown a schematic viewof an evacuated, linear, particle accelerator 10 adapted foraccelerating a particle beam.

While the particle accelerators shown and described below are especiallydesigned for accelerating a beam of electrons, the features of thepresent invention are equally applicableto apparatus for acceleratingbeams of other particles such as, for example, positrons. Also, the apparatus is adaptable for use with both pulsed'and con- :inuous beams.

The particle accelerator 10 includes a particle source 11 such as anelectron gun and an accelerating structure 7 including, .for example, abunchin-g section 13, a uniform accelerating section 14 and aterminating section15 all )f which are adapted to pass the electron beamdirected ;hereinto bythe source 11. Typically for pulsed operation of aparticle accelerator I squared, high voltage pulse is applied toelectrodes vithin the gun assembly 11 which serve to pulse the emis-:ion of the electron beam.

A high power, radio frequency source (not shown), For example, aklystron amplifier, serves to provide to he accelerating structure bymeans of an input coupler [2 peak radio frequency beam accelerationpower as, by vay of illustration, on the order of 1.75 megawatts at a:ert-ain high frequency as of, for example, 2,998 mega- :ycles. The highfrequency source is pulsed on in syn- :hronism with the pulses appliedto the electrodes of he electron gun.

In the bunching section 13 which can typically be a lisc loadedwaveguide certain dimensions, such as the.

vaveg'uide inside diameter, the beam aperture diameter, he disc spacingand the disc thickness, can be varied [long the length thereof wherebythe electron beam iassing therethrough will be velocity modulated suchhat the beam willbe formed into bunches of electrons s is passes intothe uniform accelerating section 14. llso the b'unching section may takea number of diferent forms, or may not even be utilized under certain:onditions.

crest of the .radiofrequency wave propagating through theacceleratingstructure. Aswthese electronv bunches pass through theuniformaccelerating,section.14, energy is continuously given up ;by theradio frequency wave to the electron bunches and the bunches are therebygreatly accelerated.

In typical'linear electron accelerators when the accelerated electronbeam emerges from an accelerating structure the particles making .up the:beam will have obtained extremely high energies such as anywhere from afew to very many .mi-llion electron volts. Also, for maximum efiiciencyof X-ray production theradio frequency power remaining in theaccelerating guide when. the traveling radio frequency Wave .has reachedthe end.

of the accelerating structure may typically be .on the order of 10% ofthe injectedradio frequency power. This power is usuallypassed throughan output, trans? former and either through. radio frequency vacuumwindow assembly to a Water cooled load-or through a waveguide to be fedbackinto the input of the accelerating structure.

The terminating section; 15 :according to the present invention and morefully described belowis positioned at the outputend of the uniformaccelerating sect-ion 14 and not only acts as' a radio frequency waveterminating section but also-as a meansaforcontinuing to accelerate theelectron beam after it enters from'the accelerating section 14 to thedesired .energy level, or leaving the entry energy unaltered or'reducingthe beam energy as required by the particular application. It is usuallydesirable to continue to impart energy to the beam in the terminatingsection.

' The external vwall of the=waveguide of. the bunching section 13, theaccelerating section 14 and the terminating section 15can,;itself,constitute the vacuum envelope for the particleacceleratorrll or, as is shown in the illustrated embodiment of thepresent invention, a vacuum envelope 16 can enclose the radio frequencywaveguide whichis then providedwith pumpout holes whereby the waveguidecan be evacuated. 1

The highenergy bunched particle beam emanating- A beam focusing solenoid17 circumscribes the accelerating structure to 'preventthe beam fromspreading as it' travels alongthe length of the .particle accelerator10. Since with the present invention ,a radio frequency wave coupler isnot provided vat the output end.of:the particle acceleratorlfl, thefocusing solenoid 17. can conveniently be slipped over the .outputiendof the; accelerating struc-x ture onto the accelerating structure. Thisis a mostdesirable advantage of :the .present inventionand reduces the 1cost andacomplexityof thevacu-um envelope 16; Another great advantage ofthis arrangement owing tothe absence of external connections is.thecapability of the acceleration structure to freely; expandand-contract within the vacuumrvessel as described in'detail below.

' Referring now to FIGS. 2 and 3 there is shown a typical embodiment ofthe: terminating section,15 of the present invention. Thestrncture ofthe terminatingv section 15.

includes a disc loaded waveguide 18 forming a slow Wave structure forthe radio frequency power in the accelerating structure and having thesame: dimensions as, the disc In either. case, the; particle beam --can,

loaded waveguide of the uniform accelerating section 14. The loadedwaveguide 18 is made up of a plurality of centrally apertured conductivedisc members 18 as of, for example, copper, each disc member 19 beingspaced from the adjacent disc member 19 by a hollow cylindrical spacerwall 21 of high, electrically resistive magnetic material such as, forexample, magnetic stainless steel.

The alternatively stacked disc members 19 and spacer walls 21 form aplurality of cavity resonators 22 coupled together through the centralapertures 23 in the disc members 19. A plurality of pump out holes 24are provided in each of the spacer walls 22 to assist in the evacuationof the waveguide 18 positioned within the envelope 16.

Waveguide cooling means such as water cooling tubes (not shown) areprovided on the exterior surface of the waveguide for cooling thewaveguide since a large amount of heat is generated in the terminatingsection wherein residual radio frequency power is attenuated. If thewall of the waveguide 18 were the vacuum envelope it could be completelysurrounded by a cooling fluid. A plurality of radial supports 25 areprovided for supporting the waveguide 18 within the envelope 16.

A radio frequency cut-01f device 26 is provided at the end of thewaveguide 18 of the terminating section 15 for preventing the passage ofradio frequency waves through the end of the waveguide 18. However, thiscut-off device 26 can be removed from the end of the guide forextracting or introducing radio frequency power during the testing ofthe accelerating structure.

The last cavity resonator 22 in the terminating section 15 provides areflect-ion for the radio frequency waves traveling along theaccelerating structure whereby the residual radio frequency wave powerat the end of the particle accelerator is reflected back down theaccelerating structure in a direction opposite to the direction of theparticle beam.

By the construction of the particle accelerator described above theaccelerating structure can be positioned within the vacuum envelope 16with only the input end of the accelerator structure 13 anchored to thevacuum envelope 16 whereby the remainder of the accelerating structureincluding the length of the bunching section 13, the acceleratingsection 14, and the terminating section is free to move lengthwisewithin the envelope 16 when the accelerating structure expands as itbecomes hot.

The particle accelerator 10 operates in the following manner. As theradio frequency waves coupled into the accelerating structure throughcoupler 12 travel through the bunching section 13 and the acceleratingsection 14 the waves impart energy to the electrons passing therethroughto bunch and accelerate the particles of the beam. From the acceleratingsection 14 the radio frequency wave propagates into the terminatingsection 15 wherein it continues to interact with the particle beam andimpart energy thereto. Furthermore, as the radio frequency wavepropagates through the terminating section it is greatly attenuated,especially by the highly resistive spacer walls 21. Thus, the radiofrequency wave continues to give up energy to the electron beam as it isbeing attenuated itself. In the last cavity resonator 22 of theterminating section 15 the radio frequency wave is reflected and travelsbackward in the terminating section 15 in the opposite direction to thedirection of the electron beam and is again attenuated as it passestherethrough. The length of the terminating section is selected suchthat the attenuating characteristics of the terminating section 15 inthe forward direction plus the attenuating characteristics of theterminating section 15 on the reflected wave traveling in the backwarddirection provide the proper amount of attenuation for the residualradio frequency wave power entering the terminating section 15 from theaccelerating section 14 in order to prevent interference by thereflected wave with the bunching and accelerating characteristics of thebunching section 13 and the accelerating section 14, and to preventundue frequency pulling of the radio frequency generator.

In a typical particle accelerator of the type herein described theelectron phase positions commence to deviate undesirably from theirdesign orbits within the buncher section when the available forwardpower is reduced by 8%. Also, the capability of the buncher section toproduce an energy focus is effected when the reflected power is at ahigh level.

Even though the terminating section 15 attenuates the residual power ofthe radio frequency wave in the forward direction, under some conditionsit is undesirable to have the residual radio frequency power at the endof the terminating section so low that the particle beam willregeneratively provide radio frequency power and will give up energy tothe radio frequency structure. According to the present invention thelength of the termination can be arranged such that the attenuation inthe forward direction can be reduced to zero if necessary but forpractical purposes this attenuation is selected such that in combinationwith the attenuation in the backward direction the level of thereflected power is sutficiently reduced whereby the bunching andaccelerating characteristics of the accelerating structure are notimpaired and the radio frequency generator is not pulled.

The following two tables show the manner in which the residual radiofrequency power in a typical particle accelerator can be attenuatedwithin the accelerating structure. The specific particle accelerator isespecially adaptable for therapy. The injected peak radio frequencypower is approximately 1.75 megawatts and electron energies on the orderof about 6 million electron volts are produced. However, much greaterenergies can be produced. The tables show the attenuatingcharacteristics of particle accelerators of equal length wherein alongthe accelerator axis the lengths of the accelerating section 14 and theterminating section 15 are varied to vary the amount of attenuationpresented to the residual radio frequency power.

*i.e., 6% of forward power.

TABLE 11' Forward Power, mw. Reflected Power, mw. Electron Distance Inems. Beam along acceler- Energy ator axis in Unleaded Loaded UnleadedLoaded (mev.)

Fig. 1

*i.e., 1.75% of forward power. The amount of attenuation presented tothe residual radio frequency power leaving the uniform acceleratingsection 14 can be varied in several different ways. At-

tenuation, I, is determined by the following formula:

wherein c is the velocity of light, Q is 21r times the ratio of theenergy stored in the cavity resonators of radio frequency propagatingstructure to the energy lost in the cavity resonators of the structureper cycle, Vg is the group velocity of the radio frequency wavetraveling along the structure and A is the free space wavelength of theradio frequency wave. Therefore, for a given operational frequency theattenuation in the terminating section 15 can be increased over theattenuation presented in the uniform accelerating section 14 bydecreasing Q and/ or Vg. Q can be decreased by decreasing the ratio ofeffective cavity volume to effective electrical surface area. Anotherway of reducing the Q of a cavity is to increase the permeability and/or resistivity.

In the embodiment described above the cavity walls of high resistivitymagnetic material, such as magnetic stainless steel greatly increasedthe attenuation of a terminating section having cavities of the samedimensions as those of the accelerating section. For example,accelerating sections having copper spacer walls would have a Q on theorder of 12,500 whereas the cavities of the terminatlng section 15described above of similar size as those of :he uniform acceleratingsection but with magnetic stainless steel walls 21 would have a Q on theorder of 1400. Even if the walls were of high resistivity non-magneticnaterial instead of magnetic material the Q would still be on the orderof 2500.

As an alternative embodiment of the present invention :he disc members19 as well as the spacer walls 21 can be made of high, electricallyresistive material to provide even greater attenuation for the radiofrequency wave n the terminating section 15. However, in such a struc-Lure the heat dissipated in the disc members 19 cannot 3e conducted outto the cooling means surrounding the Waveguide '18 as easily as in theembodiment described rbove wherein the disc members 19 are made ofhighly :onductive material such as copper. In this present em- Jodimentcooling means such as water channels can be provided into the discmembers 19 to more adequately 2001 these disc members 19.

Referring now to FIG. 4, there is shown another em- Jodiment of thepresent invention wherein a plurality of grooves 27 providing an unevensurface are provided iCl'OSS the current path along the surface of thedisc mem- Jers -19' and the spacer walls 21' whereby the residual -adiofrequency power in the waveguide can be attenuated .o a much' greaterdegree within the same axial length of waveguide. :urrent path decreasethe cavity Q by decreasing the ratio )f effective cavity volume toeffective electrical'surface area. The cavities of a terminationconstructed in such The grooves cut in this manner across the i absorbedperunitlengththan in the acceleratingwaveguide and even though thestructure is cooled the temperature of the structure will rise,.the1cavities will expand, and .the waveguide wave length of the cavitieswill change-thereby causing a, phase shift between the electron beam andthe, radio frequency wave. This results in undesirable contributions toelectron beam energy spread.

It is therefore advantageous to have .a very high attenuation factor sothatthe termination can be very short in length thereby considerablyminimizing the spectrum spreading contribution to the electron beam dueto 0p erational fluctuations.

If; the material 'of the terminating section is made of great for agiven rise in temperature, then the phase; shift will be even less.

Still a further alternative embodimentof the; present invention isillustrated in FIG. 51Which shows the last cavity resonator 28 ofthe.uniformaccelerating section 14 and the firstcavity resonator 29 ofthe terminating section 15"; In this; embodiment the central apertures23" of the disc members 19 are-smaller than the central apertures 32 0fthe. disc members 31 in the uniform accelerating section whereby thecavity resonators in the. terminating section propagate a radiofrequency wave therethrough With a'reduced group velocity .than;do thecavity resonators of the acceleratiugsection14".

As still a further embodiment of the present invention, the desirablylow Q of the cavities in the terminating sec.- tion can be achievedwhile'still utilizing properly treated cavity spacer walls and discmembers of the same dimensions and samematerials as. those of theuniform accelerating section." This greatly facilitates-the fabricationof the particle generator. Referring to FIG. .6, a ter-. minatingsection 33 secured to the end of a uniform ac-. celerating' section 30includes a plurality'of cavity re-; sonators 34 comprised of alternatelystacked apertured disc members 35 such: as copper and spacer walls 36 isuch as copper, adjacent cavities being coupled through creasing theattenuation. A good material for thispurpose is metal powder soldunderthe trademark tKanthal.

l manner with disc members 19,115 of copper, and spacer In many ways,such a material is ideal for increasing, the attenuation of the cavitystructure. I has a highresistivity p and high magnetic permeability ,u..Furthermore, the Kanthal sprayed on the cavity surfaces leaves a veryuneven surface whereby the ratio of cavity volume to effective surfacearea istreduced. A cavity of the same dimensions as those of thecavities described :above and 9 with copper spacer walls sprayed withKanthal has a Q of 300. An even lower Q can be provided by spraying theKanthal on the disc members 35 as well as the spacer walls 36.

Typically as shown in the FIG. 6, the amount of Kanthal provided insuccessive cavities along the length of the terminating section 33 isgradually increased such that a very large thermal discontinuity willnot be provided at the front end of the terminating section 33 of theaccelerating structure due to an extremely large difference between theattenuation in the last cavity of the uniform accelerating section 30and the attenuation in the first cavity of the terminating section 33.

Also Kanthal can be applied to portions of the terminating section byfirst sandblasting the region where the Kanthal is to be applied. Thispresents an even rougher surface and permits the Kanthal to stick to thestructure better.

Other materials besides Kanthal that can be sprayed onto the terminatingstructure are high loss materials such as stainless steel, iron, Kovar,etc.

The following table shows the manner in which the residual radiofrequency power can be attenuated in a typical particle accelerator ofthe construction shown in FIG. 6.

nators adapted to pass a particle beam therethrough and to propagate aradio frequency wave for interaction with and acceleration of saidparticle beam, said cavity resonators of said termination sectionadapted to progagate a radio frequency wave therethrough with a reducedgroup velocity than said cavity resonators of said accelerating sectionwhereby any residual power of a radio frequency wave emanating from saidaccelerating section is greatly attenuated in said termination section.

2. A particle accelerating structure comprising, in combination, anaccelerating section including a disc loaded waveguide comprised of aplurality of coupled cavity resonators adapted to pass a particle beamtherethrough and to propagate a radio frequency wave for interactionwith and acceleration of said particle beam, and a collinear terminatingsection including a disc loaded waveguide comprised of a plurality ofcoupled cavity resonators adapted to pass a particle beam therethroughand to propagate the radio frequency wave for interaction with andacceleration of said particle beam, the apertures in the discs of saidterminating section being smaller than the apertures in the discs ofsaid accelerating section whereby any residual power of a radiofrequency wave emanating from said accelerating section is greatlyattenuated within said terminating section.

*0.94% of forward power.

VSWR at RF generator without ferrite isolator and without beam loading:1 .31

VSWR at RF generator without ferrite isolator and with 100 ma. pk.loading: 1.186

VSWR at RF generator with 5 db isolation and with 100 ma. pk.loading=1.l06

VSWR at RF generator with 10 db isolation and with 100 ma. pk.loading=1.060

Alternatively, the attenuation provided by the terminating section 15 tothe residual radio frequency power at the end of the uniformaccelerating section can be increased by loading the cavity resonatorsof the terminating section with loading members such as resistive coatedceramic members or by increasing the thickness of the disc members inthe terminating section over the thickness of those in the uniformaccelerating section.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A particle accelerating structure comprising, in combination, anaccelerating section including a disc loaded waveguide comprised of aplurality of coupled cavity resonators adapted to pass a particle beamtherethrough and to propagate a radio frequency wave for interactionwith and acceleration of said particle beam, and a collinear terminatingsection including a disc loaded waveguide comprised of a plurality ofcoupled cavity reso- 3. A particle accelerator comprising, incombination, an accelerating section including a disc loaded waveguidecomprised of a plurality of coupled cavity resonators adapted to pass aparticle beam therethrough and to propagate a radio frequency wavetherethrough for interaction with and acceleration of said particlebeam, and a collinear terminating section including a dis-c loadedwaveguide comprised of a plurality of coupled cavity resonators adaptedto pass a particle beam therethrough and to propagate a radio frequencywave therethrough for interaction with and acceleration of said particlebeam, the discs of said terminating section being thicker than the discsof said accelerating section whereby residual power of a radio frequencywave emanating from the end of said accelerating section is greatlyattenuated in said terminating section.

4. A particle accelerating apparatus comprising, in combination, anaccelerating waveguide means for propagating a radio frequency wave forinteraction with and acceleration of a particle beam passingtherethrough from an input end at which the particles in the beam areintroduced at an initial velocity to an output end at which theparticles in the beam are discharged at an accelerated velocity higherthan said initial velocity and a collinear termination section having atermination input end located at the output end of said acceleratingwaveguide means for receiving the accelerated particles discharged fromthe output end of said accelerating waveguide means and a terminationoutput end for passing the accelerated particles for utilization, saidcollinear termination section including cavity resonators apertured forpassing particles from the output end of said accelerating waveguide tothe termination ouput end in interacting relationship with the radiofrequency wave traveling from the outputend of said acceleratingwaveguide through said termination section, said cavity resonatorshaving wall means for providing high attenuation to the radio frequencywave pass ing therethrough with said wall means positioned to insuretransfer of energy from the radio frequency wave to the particlestraveling along the length of said termination section whereby withinthe termination section the radio frequency wave is attenuated as itgives up energy to accelerate the particles.

5. The particle accelerating apparatus in accordance with claim 4including means at the termination output' end for causing residualradio frequency power remaining at said termination to be reflected backtoward said termination input end whereby the radio frequency wavereflected at the termination output end is attenuated while travelingthrough said termination section in the backward direction.

6. The particle accelerating apparatus in accordance with claim 4wherein said termination section includes a loaded waveguide defining aplurality of cavity resonators and at least a portion of the walls ofsaid cavity 7 resonators including a high, electrically resistivematerial for attenuating the radio frequency wave travelingtherethrough.

7. The particle accelerating apparatus in, accordance.

with claim 4 characterized further in that said termination sectionincludes a loaded waveguide defining a plurality of cavity resonators,at least a portion of said cavity resonators being provided with anuneven surface which increases the attenuating characteristics of saidresonators to radio frequency waves for attenuating radio frequencywaves traveling therealong.

8. The particle acceleraing apparatus in accordance. with claim 7characterized further in that said uneven surface is Kanthal sprayedonto the surface of a portion of said cavity resonators.

9. A particle accelerating structure comprising, in combination, anaccelerating section including a disc loaded waveguide constructed topass a particle beam therethrough from an input end at which theparticles in the beam, are introduced at an initial velocity to anoutput end at which the particles in the beam are discharged at anaccelerated velocity higher than said initial velocity and to propagatea radio frequency wave for interaction with and acceleration of saidparticle beam from said input end to said output end, means fordirecting a particle beam into the input end of said acceleratingsection for acceleration therealong, means for introducing into saidaccelerating section atthe input end thereof a radio frequency wavefor'interac-tion with and acceleration of said particle beam, acollinear termination section at the output end of said acceleratingsection including a disc loaded waveguide adapted to pass a particlebeam and to propagate a radio frequency wave from a termination inputend adjacent the output end of said accelerating section to atermination output end for beam-wave interaction and acceleration ofsaid particle beam, the attenuation of said termination sectionpresented to the radio frequency wave therein being greater than theattenuation of said accelerating section presented to .a radio frequencywave passing therethrough for greatly attenuating the residual power ofradio frequency waves passing from the output end of said acceleratingsection through said termination section while maintaining a high axialaccelerating electric field in at least the initial portion of saidtermination section, and means for passing accelerated particles out ofsaid termination output end for utilization.

10. A particle accelerating structure comprising, ,in combination, anaccelerating section including a disc loaded waveguide means having aninput end at which the particles in the beam are introduced at aninitial velocity and an output end at which the particles in the beamare discharged at an accelerated velocity higher than said 12 initialvelocity and comprised of a plurality. of coupled cavity resonators forpassing a particle beam therethrough from said input end ,to saidoutput. end and propagating a radio-frequency wave for interaction withand ac.

celeration of said particle beam from said input end to said output end,means for directing a particle beam into said input end of saidaccelerating section for accelera tion therealong, means for introducinga radio frequency wave into said acceleratingt section for interactionwith and acceleration of said'particle beam therein, a collineartermination section having a termination input end and aterminationoutput iend,'-said termination input:

end located adjacent the output end of said accelerating section,saidtermination section-including ,a disc loaded waveguide comprised 30fa plurality ofcoupled cavity resonators for passing a particleibeamtherethrough from said termination input end to said termination output;end and propagation of radio frequency wavesthere-- through forinteraction with and acceleration :of said particle beam while saidradiofrequency wavesttravel.

from said termination input end .to said termination output end/the Q ofthe cavity resonators of said termina tion section being much lessthanthe-Q ofthe cavity resonators of said accelerating section whereby anyresidual power of aradio frequency, waveemanating from.

said accelerating section is greatly attenuated in said terminationsection, and means for passing accelerated particles out of thetermination output end .for utilization.

11..The methodof accelerating a beam of charged particles to a highvelocity comprising the steps of:

passing a beam of charged .particles in synchronism with a radiofrequency wave vwhereby energy is' given up by the wave to acceleratethe particles of the beam;-then after the beamis partially accelerated,greatlyv attenuating the. residualpower of.

the radio frequency wave along-thebeam path while providing continuous,simultaneous interaction between the ,wave and the particles of saidbeamto continue to accelerate the beam .of charged particles; thenreflecting the radio frequency wave back in the opposite direction tothe direction of said accelerating beam; and again greatly attenuatingthe residual power of theiradio frequency wave as it travels along thebeam path in the direction opposite,

to the directionof the accelerating beam.

12.; A termination for particle accelerating .structures for terminatingradio frequency power adjacent the output end of the acceleratingstructure atawhich the accelerated beam of chargedparticle emergescomprising, in combination, means for coupling radio frequency powerinto the terminatiom'a plurality of apertured discs, a plurality ofhollow, cylindrical spacer Walls, one of said spacer walls interposedbetween eachadjacent pair of apertured discs whereby said discs and saidwalls form cavity resonators adapted to propagate a radio frequency wavecoupled into said termination at a first end thereof; a highlyresistantxmaterial provided onnat 'least certain of the surfaces of saidcavity resonators, said cavity resonators phase tuned vfor propagatingthe; radio frequency wave, the amount of said resistant materialprovided in each of said cavities increasing from said first end of saidterminationto the second end of said termination,

the cavity resonator. at the second end .of said termination positionedas to reflect any residual power of saidradio frequency .wave travelingtherein back toward said first end of said terminationsection wherebytherradio fre.

quency wave is attenuated both as ittravels in the forward directionfromsaid first end toxsaid second end of said termination .and in thebackward direction from said second end to said first end of 'saidtermination and means fo'rconnecting said first end of said terminationsection to the output end of an accelerating structure at which theaccelerated beam of charged particles emerges.

(References on following page) 13 14 References Cited by the Examiner2,993,143 7/1961 Kelliher et a1. 3155.42 X 3 018448 1/1962 Warnecke315-3.6 X UNITED STATES PATENTS I 8/1937 King X 3,068,425 12/1962 Boutetet a1 315 5.42 X 4/1946 Sloan 313-55 X 5 FOREIGN PATENTS 6/1951 Pierce31355 X 767,506 2/1957 Great Britain. 9/1953 Woodyard 313-55 X 9/1956Eldredge et a1. 333-31 DAVID I GALVIN, Primary Examiner. E4132; g f315-142 GEORGE WESTBY, ARTHUR GAUSS, JOHN W.

61516 HUCKERT Examiners 3/1960 Marchese 315-393 X 10 5/1961 Kompfner3153.5 C. O. GARDNER, R. SEGAL, Assistant Examiners.

11. THE METHOD OF ACCELERATING A BEAM OF CHARGED PARTICLES TO A HIGHVELOCITY COMPRISING THE STEPS OF: PASSING A BEAM OF CHARGED PARTICLES INSYNCHRONISM WITH A RADIO FREQUENCY WAVE THEREBY ENERGY IS GIVEN UP BYTHE WAVE TO ACCELERATE THE PARTICLES OF THE BEAM; THEN AFTER THE BEAM ISPARTIALLY ACCELERATED, GREATLY ATTENUATING THE RESIDUAL POWER OF THERADIO FREQUENCY WAVE ALONG THE BEAM PATH WHILE PROVIDING CONTINUOUS,SIMULTANEOUS INTERACTION BETWEEN THE WAVE AND THE PARTICLES OF SAID BEAMTO CONTINUE TO ACCELERATE THE BEAM OF CHARGED PARTTICLES; THENREFLECTING THE RADIO FREQUENCY WAVE BACK IN THE OPPOSITE DIRECTION TOTHE DIRECTION OF SAID ACCELERATING BEAM; AND AGAIN GREATLY ATTENUATINGTHE RESIDUAL POWER OF THE RADIO FREQUENCY WAVE AS IT TRAVELS ALONG THEBEAM PATH IN THE DIRECTION OPPOSITE TO THE DIRECTION OF THE ACCELERATINGBEAM.